THE EARLIEST CRUSTACEA 39 



the nearest approach to an ancestral form met with in 

 the fossil state. Now that the possession of true 

 antennae is certain, it is reasonable to suppose that 

 the Trilobites represent an early Class of the Aceratous 

 branch which had not yet become Aceratous. They 

 are thus of the deepest interest in helping us to under- 

 stand the origin of the antennaless branch, not by the 

 ancestral absence, but by the loss of true antennae which 

 formerly existed in the group. But the Trilobites did 

 not themselves originate the other Classes, at any rate 

 during Palaeozoic times. They represent a large and 

 dominant Class, presenting more of the characters of 

 the common ancestor than the other Classes ; but the 

 latter had diverged and had become distinct long before 

 the earliest fossiliferous rocks ; for we find well-marked 

 representatives of the Crustacea in Cambrian, and of 

 the Arachnida in Silurian strata. The Trilobites, more- 

 over, appear in the Cambrian with many distinct and 

 very different forms, contained in upwards of forty genera, 

 so that we are clearly very far from the origin of the 

 group. 



Of the lower group of Crustacea, the Entomostraca, 

 the Cirripedes are represented by two genera in the 

 Silurian, the Ostracodes by four genera in the Cambrian 

 and over twenty in the Silurian ; of these latter, two 

 genera (Cythere and Bairdia) continue right through 

 the fossiliferous series and exist at the present day. 

 Remains of Phyllopods are more scanty, but can be 

 traced in the Devonian and Carboniferous rocks. The 

 early appearance of the Cirripedes is of especial interest, 

 inasmuch as the fixed condition of these forms in the 

 mature state is certainly not primitive, and yet, never- 

 theless, appears in the earliest representatives. 



The higher group, the Malacostraca, are represented 

 by many genera of Phyllocarida in the Silurian and 

 Devonian, and two In the Cambrian. These also afford 

 a good example of the imperfection of the record, inas- 

 much as no traces of the group are to be found between 

 the Carboniferous and our existing fauna in which it 

 is represented by the genus Nebalia. The Phyllocarida 



